Communication device for receiving and transmitting OFDM signals in a wireless communication system

ABSTRACT

A communication device for receiving and transmitting OFDM signals in a wireless communication system, in which each OFDM signal is composed of a plurality of subcarrier signals each being assigned to a respective transmission channel of a communication system, comprising a diversity antenna; an examination unit for examining, individually for each antenna element, at least one subcarrier signal of an OFDM reception signal received by a respective antenna element and for gaining, from the result of such subcarrier signal examination, attenuation information on attenuation properties of at least some of the transmission channels associated to the respective antenna element; and an amplitude adjustment unit adapted for adjusting, individually for each antenna element, the amplitude of at least one subcarrier signal of an OFDM transmission signal to be transmitted from a respective antenna element in accordance with attenuation information.

This is a continuation of application Ser. No. 09/935,925, filed Aug.23, 2001 now U.S. Pat. No. 7,085,223, which claims priority to Europeanapplication serial number 00 118 418.3, filed Aug. 24, 2000, theentirety of which is incorporated herein by reference.

The present invention relates to a communication device for receivingand transmitting OFDM signals in a wireless communication system.

In wireless OFDM communication systems a communication device, as e.g. abase station, communicates with another communication device, as e.g. amobile terminal, over a wireless communication link using OFDM signals.OFDM (orthogonal frequency division multiplex) is a multi carriermodulation method wherein information to be transmitted is mapped (e.g.by phase shift keying) onto a plurality of orthogonal subcarrierssignals of different frequencies which are subsequently combined into anOFDM signal. Each subcarrier frequency defines a transmission channel inwhich information can be transmitted over the communication link. Formore background information on OFDM it is referred, for example, to K.David, T. Benkner: “Digitale Mobilfunksysteme”, B. G. Teubner Stuttgart,1996, S. 174-176.

The communication link causes undesired level fluctuations anddistortion of the transmitted OFDM signal, e.g. due to fast fading ordelay spread. Diversity methods can alleviate the adverse effects offading. Using a plurality of antenna elements spaced apart at a certainminimum distance allows, by suitably combining reception signalsreceived by the various antenna elements, to reliably recover thebaseband information sent from another communication device even iffading occurs on one or more of the transmission paths across thecommunication link (receiver diversity). Transmitting one and the sametransmission signal mutually delayed from several antenna elementsallows to create a beam sharpened antenna pattern and to increase thereceived signal power at the receiver side (transmitter diversity). Formore background information on receiver and transmitter diversity see,for example, EP 0 881 782 A2.

In receiving OFDM signals by array antennas or any other diversityantennas, heavy amplitude fading may occur to the entire OFDM signal atone (or more) of the antenna elements of the diversity antenna. However,it has been found that often amplitude fading does not occur to all ofthe subcarrier signals of an OFDM signal to the same extent. In somecases it can be observed that only particular ones of the subcarriersignals are subject to amplitude fading while other subcarrier signalsand possibly even the OFDM signal as a whole do not show any severeamplitude fading.

This can be seen in FIG. 1 which schematically shows an example for thefrequency selectivity of fast fading. In FIG. 1, the horizontal abscissaindicates the subcarrier number n and the vertical ordinate indicatesthe signal amplitude A of the aligned preamble OFDM subcarriers, whereineight antenna elements are positioned to form a circle. Note that thedip in the center is not due to any fading but to the fact that in theBran Hiperlan2 approach being the base for the simulation the centersubcarrier is not used. By assuring that there is no DC component in thebaseband signal the demodulation complexity is reduced. However, FIG. 1clearly shows a frequency selective fading which might appear dependingon the application environment.

Having discovered these effects, it is the object of the presentinvention to reduce the energy consumption at the transmitter side bytaking advantage of the above insight. Furthermore it is the object tooptimize the signal energy at the receiving antenna on the same basis.

According to a first aspect, the invention provides a communicationdevice for receiving and transmitting OFDM signals in a wirelesscommunication system, in which each OFDM signal is composed of aplurality of subcarrier signals each being assigned to a respectivetransmission channel of the communication system, the communicationdevice comprising:

-   -   diversity antenna means including a plurality of antenna        elements,    -   examination means adapted for examining, individually for each        antenna element, at least one subcarrier signal of an OFDM        reception signal received by a respective one of the antenna        elements and for gaining, from the result of such subcarrier        signal examination, attenuation information on attenuation        properties of at least some and preferably all of the        transmission channels associated to the respective antenna        element, and    -   amplitude adjustment means adapted for adjusting, individually        for each antenna element, the amplitude of at least one        subcarrier signal of an OFDM transmission signal to be        transmitted from a respective one of the antenna elements in        accordance with the attenuation information, such as to give a        higher amplitude to the subcarrier signal of the OFDM        transmission signal when the attenuation information indicates a        lower attenuation of the associated transmission channel, and to        give a lower amplitude to the subcarrier signal of the OFDM        transmission signal when the attenuation information indicates a        higher attenuation of the associated transmission channel.

The inventive solution takes benefit of the above observation (i.e.subcarrier dependence of amplitude fading) and reduces the amplitude ofthose subcarrier signals of the OFDM transmission signal at a particularantenna element which are expected to be subject to amplitude fading. Asit is highly improbable for a transmission channel to be heavilydisturbed at the same time at all antenna elements, amplitude reductionof a particular subcarrier signal being transmitted from one of theantenna elements will have—if at all—only negligible effect on theability to reliably recover the information hidden in this particularsubcarrier signal on the receiver side because the same subcarriersignal still will be transmitted from other antenna elements at normalor just slightly reduced amplitude. By giving a minor role to thosesubcarrier signals which, due to fading, will not reach the receivingcommunication device at a reasonable amplitude level, waste of uselessenergy can be avoided and signal interference caused by such uselessenergy at other receiving communication devices can be reduced. Anevaluation as to which of the subcarrier signals of the OFDMtransmission signal are likely to be subject to amplitude fading andtherefore should be reduced in amplitude is done by examining an OFDMreception signal sent from the communication device intended to beaddressee of the OFDM transmission signal. This signal examination isdone individually for each antenna element.

It is conceivable that the amplitude adjustment means are adapted tosuppress the subcarrier signal of the OFDM transmission signal to betransmitted from the respective antenna element when the attenuationinformation indicates that the attenuation of the correspondingtransmission channel exceeds a predetermined threshold.

In a preferred embodiment of the invention, the communication devicecomprises memory means for storing data representing a predeterminedreference signal, and the examination means are adapted for comparing apredetermined portion of the subcarrier signal of the OFDM receptionsignal with the reference signal and for gaining the attenuationinformation from the result of such comparison. In particular, thereference signal may comprise a reference preamble symbol, and theexamination means may be adapted for comparing a preamble portion of thesubcarrier signal of the OFDM reception signal with the referencepreamble symbol.

The examination means may further be adapted for gaining, from thesubcarrier signal examination, phase shift information on phase shiftproperties of at least some of the transmission channels associated tothe respective antenna element. Then, phase adjustment means may beprovided which are adapted for phase adjusting, individually for eachantenna element, at least one subcarrier signal of said OFDMtransmission signal in accordance with said phase shift information.

According to a second aspect, the invention provides a method foroperating a communication device for receiving and transmitting OFDMsignals in a wireless communication system, in which each OFDM signal iscomposed of a plurality of subcarrier signals each being assigned to arespective transmission channel of the communication system, thecommunication device comprising diversity antenna means including aplurality of antenna elements, the method comprising the steps of:

-   -   examining, individually for each antenna element, at least one        subcarrier signal of an OFDM reception signal received by a        respective one of the antenna elements and gaining, from the        result of such subcarrier signal examination, attenuation        information on attenuation properties of at least some and        preferably all of the transmission channels associated to the        respective antenna element, and    -   adjusting the amplitude of at least one subcarrier signal of an        OFDM transmission signal to be transmitted from a respective one        of the antenna elements in accordance with the attenuation        information, such as to give a higher amplitude to the        subcarrier signal of the OFDM transmission signal when the        attenuation information indicates a lower attenuation of the        associated transmission channel, and to give a lower amplitude        to the subcarrier signal of the OFDM transmission signal when        the attenuation information indicates a higher attenuation of        the associated transmission channel.

The above method may comprise the step of suppressing the subcarriersignal of the OFDM transmission signal to be transmitted from therespective antenna element when the attenuation information indicatesthat the attenuation of the corresponding transmission channel exceeds apredetermined threshold.

In case that the communication device comprises memory means for storingdata representing a predetermined reference signal, the method accordingto the present invention may comprise the step of comparing apredetermined portion of the subcarrier signal of the OFDM receptionsignal with the reference signal and gaining the attenuation informationfrom the result of such comparison. In particular, the reference signalmay comprise a reference preamble symbol; then, a preamble portion ofthe subcarrier signal of the OFDM reception signal may be compared withthe reference preamble symbol.

Additionally, there may be provided the step of gaining, from saidsubcarrier signal examination, phase shift information on phase shiftproperties of at least some of the transmission channels associated tothe respective antenna element and phase adjusting at least onesubcarrier signal of the OFDM transmission signal in accordance with thephase shift information.

According to yet another aspect, the invention provides a computerprogram which performs, when executed by a processor of a communicationdevice, the method described above.

The present invention is explained in more detail in the followingdescription in relation to the enclosed drawings, in which:

FIG. 1 is a schematic graph showing the magnitude of the alignedpreamble OFDM subcarriers as a function of the subcarrier number,

FIG. 2 is a schematic block diagram of a communication device accordingto an embodiment of the present invention,

FIG. 3 schematically shows how a frequency domain channel estimation isperformed within the communication device of FIG. 2, and

FIG. 4 a through 4 c schematically show the generation of a complexweight coefficient.

The communication device illustrated in FIG. 2 may be, for example, abase station or a mobile terminal of a wireless TDMA-TDD communicationsystem employing a time division multiplex access (TDMA) and timedivision duplex (TDD) technique for OFDM signal communication betweenthe various communication devices of the communication system. Thecommunication device comprises all components necessary for processingreceived OFDM signals and generating OFDM signals to be transmitted. Inparticular, the communication device comprises a diversity antenna, e.g.an array antenna, generally designated by 10. The diversity antenna 10is comprised of a plurality of mutually spaced antenna elements 12 onlytwo of which are shown. An OFDM signal sent from a distant mobileterminal (not shown) or another type of communication device is receivedby each antenna element 12. In general, there will be a phase differencebetween the incoming OFDM signals at the different antenna elements 12.The received OFDM signals are individually processed by thecommunication device according to the present invention. Particularly,the received OFDM signals pass through down-conversion andanalog-to-digital conversion means 14, Fast Fourier Transforming (FFT)means 16, demodulation means 18 and subsequent processing means (notshown in detail), such as a deinterleaver, a channel decoder and a voicecodec, which allow to recover the baseband information transmitted fromthe remote station or terminal. At some point in the signal processingchain there will be provided combining means for combining the differentreceived OFDM signals according to a suitably selected combiningtechnique which is per se well-known in the art.

In case the communication device wants to transmit an OFDM transmissionsignal to the remote station or terminal, this OFDM transmission signalis generated by transmission signal generation means 20 (which includesuch functions as channel coding, interleaving and modulation) and issubsequently supplied to Inverse Fast Fourier Transforming (IFFT) means22 and up-conversion and digital-to-analog conversion means 24. The OFDMtransmission signal then is transmitted from each antenna element 12with a phase difference from one antenna element 12 to the next. Thephase difference at transmission is determined based on the phaserelationship between the incoming signals at the different antennaelements 12 at signal reception, so as to enhance signal power at theremote station or terminal and to reduce signal power at other places.

An OFDM signal is composed of a plurality of superposed subcarriersignals having different subcarrier frequencies. Channel estimationmeans 26 within the communication device according to the presentinvention determine an attenuation value for each subcarrier signal(better: for each transmission channel associated to a respectivesubcarrier frequency) of the received OFDM signal individually for eachantenna element 12. The attenuation value gives a measure for theattenuation that the respective subcarrier signal was subjected toduring its transmission from the remote station or terminal to therespective antenna element 12 of the communication device according tothe present invention. Such attenuation may be caused e.g. by fast andslow fading. To determine the attenuation values, the channel estimationmeans 26 compare a preamble portion of at least one of the subcarriersignals with a known reference preamble symbol prestored in a memory 28.Particularly, the channel estimation means 26 compare the magnitudes ofthe preamble portion of the subcarrier signal and the Fouriertransformed version of the reference preamble symbol and calculate amagnitude ratio (see FIG. 3). If necessary, the channel estimation means26 perform a phase alignment of the preamble portion of the subcarriersignal with regard to the reference preamble symbol before carrying outthe magnitude comparison.

Preferably, the channel estimation means 26 calculate the magnituderatio only for a limited number of subcarrier signals out of the totalnumber of subcarrier signals making up the OFDM reception signal at arespective one of the antenna elements 12. The channel estimation means26 then determine the attenuation values for the remaining subcarriersignals from the calculated magnitude ratios by estimation, e.g. byinterpolation or filtering. In this way, attenuation values for everytransmission channel associated to a respective one of the antennaelements 12 can be obtained. However, it is to be understood that thechannel estimation means 26 may be adapted to calculate the magnituderatio for all of the subcarrier signals.

The channel estimation means 26 supply the attenuation values thusdetermined to signal adjustment means 30 which determine for eachattenuation value a corresponding amplitude adjustment factor. Thus, anamplitude adjustment factor is determined in relation to eachtransmission channel associated to a respective antenna element 12. Eachamplitude adjustment factor is applied, at a multiplication point 30, tothe respective subcarrier signal of the OFDM transmission signal to betransmitted from the respective antenna element 12. In this way, theamplitudes of the subcarrier signals of the OFDM transmission signal areindividually adjusted according to the attenuation conditions of thecorresponding transmission channel. Particularly, the amplitudeadjustment is such that a lower attenuation value results in a highercorresponding amplitude adjustment factor and thus in a higher amplitudeof the corresponding subcarrier signal at the respective antenna element12, and vice versa. There may be chosen a linear relationship betweenthe magnitude ratio determined in relation to a particular transmissionchannel and the corresponding amplitude adjustment factor.Alternatively, a non-linear relationship may be chosen for thisrelationship. For example, the relationship may be chosen such that,when the magnitude ratio is below a predetermined threshold, thecorresponding subcarrier signal at the respective antenna element 12 issuppressed. And if the magnitude ratio is above the threshold, thecorresponding subcarrier signal is given a predetermined constantamplitude. In general, the choice of a suitable relationship between themagnitude ratio (i.e. the attenuation value) and the amplitudeadjustment factor will be readily available to a person skilled in theart.

The above amplitude adjustment which, if necessary, is performed foreach antenna element 12 individually on each subcarrier signal to betransmitted from the respective antenna element 12 allows to avoidtransmission of useless energy on those transmission channels which haveproved to be heavily disturbed by amplitude fading, thereby reducingpower consumption of the communication device itself and liability tointerference at other receiving communication devices due totransmission of meaningless energy.

The channel estimation means 26 may further determine, individually foreach transmission channel of each antenna element 12, a phase differencevalue representative of the phase shift that the corresponding receivedsubcarrier signal was subjected to during its transmission from theremote station or terminal to the respective antenna element 12 of thecommunication device according to the present invention. To this end,the channel estimation means 26 may determine the relative phasedifference between the preamble portion of the respective subcarriersignal and the prestored reference preamble symbol mentioned above.Again, the channel estimation means 26 may calculate the relative phasedifference individually for every subcarrier signal, or only for aselected group of subcarrier signals followed by an estimation processfor the remaining subcarrier signals. From the phase difference valuesthus determined by the channel estimation means 26 the signal adjustmentmeans 30 then determine suitable phase adjustment factors to be appliedto the subcarrier signals of the OFDM transmission signal, so ascompensate for the relative phase shifts that occur to the subcarriersignals transmitted from the different antenna elements 12 on their wayto the remote station or terminal.

The amplitude adjustment factor and the phase adjustment factor for aparticular subcarrier signal associated to a particular antenna element12 may be represented by a complex weight coefficient which is appliedto the respective transmission subcarrier signal. FIGS. 4 a to 4 c showthree possibilities for generating the weight coefficient. In all threecases, the phase of the weight coefficient equals the phase determinedby the channel estimation means 26 for the corresponding receivedsubcarrier signal but has opposite sign. The magnitude of the weightcoefficient can be equal (FIG. 4 a) or proportional (FIG. 4 b) to thesubcarrier magnitude determined by the channel estimation means 26.Alternatively, the magnitude of the weight coefficient can be the resultof a non-linear function, such as a threshold or square function, whichhas the subcarrier magnitude determined by the channel estimation means26 as its input (FIG. 4 c).

One of the objects of the present invention is to reduce unnecessaryenergy consumption at the transmitter. Furthermore, as will be shown inthe following on the basis of a mathematical representation, the energyat the receiving antenna can be maximized when applying the concept ofthe present invention.

To maximize the total OFDM signal energy at the receiver, the amplituderatio and phase at each transmitter antenna should be optimized. Now, wethink about the j^(th) subcarrier. M OFDM signals coming from Mdifferent transmitter antennas. They are already weighted by valuew_(jk). The combined signal can be written like

$\sum\limits_{k = 0}^{M - 1}{a_{jk}w_{jk}^{*}}$

Here, “*” is conjugate. a_(jk) is complex channel expression of j^(th)subcarrier of OFDM signal comes from k^(th) antenna. In vectorexpression, combined OFDM signal can be written asa_(j)w′_(j)where a_(j)=[a_(j0), a_(j1), . . . , a_(jM)] is channel vector andw_(j)=[w_(j0), w_(j1), . . . , w_(jM)] is weight vector for j^(th)subcarrier. Here, “′” is Hermitian transpose (conjugate transpose).

Power of combined signal will be(a _(j) w′ _(j))²=(a _(j) w′ _(j))′(a _(j) w′ _(j))=(w _(j) a′ _(j))(a_(j) w′ _(j))=w _(j) A _(j) w′ _(j)

The matrix A is a Hermitian matrix defined as

$A_{j} = {{a_{j}^{\prime}a_{j}} = {\begin{bmatrix}a_{j0}^{*} \\a_{jM}^{*}\end{bmatrix}\left\lbrack {a_{j0}a_{jM}} \right\rbrack}}$

To maximize combined j^(th) subcarrier of OFDM signal, it is said thatweight vector, w_(j), should be chosen to proportional of Eigenvector ofmaximum Eigenvalue of the matrix A_(j).

In the other hand, any Hermitian matrix H can be expressed like

$H = {\sum\limits_{j}{\lambda_{j}v_{j}^{\prime}v_{j}}}$where λ is Eigenvalue and ν is Eigenvector. If the expression of thematrix A_(j) is compared with this expression of Hermitian matrix H, itcan be seen that the matrix A_(j) has only one Eigenvalue which is notzero, naturally maximum Eigenvalue, and its eigenvector is a_(j).Eigenvalue of the matrix A_(j) is inner product of a_(j)a′_(j) andEigenvector is usually normalized like a_(j)/(a_(j)a′_(j))^(1/2).

Again, to maximize the combined j^(th) subcarrier of OFDM signal, weightvector, w_(j), should be proportional to vector a_(j).

For other subcarriers, set of complex channel expression of each channel(antenna), channel vector, shall be weighting vector. Complex channelexpression of each antenna and subcarrier is known at channel estimationmethod.

(Normalization of weight vector is not necessary, because ratio betweenantennas and subcarriers are important.)

1. A communication device for transmitting OFDM signals in a wirelesscommunication system, the communication device comprising: antenna meanshaving a plurality of antenna elements for transmitting said OFDMsignals to a receiver device through a plurality of transmissionchannels in said wireless communication system; means for generatingcomplex weight coefficients which are respectively applied to said OFDMsubcarrier signals to be transmitted; and means for controllingamplitude and/or phase of each of said OFDM subcarrier signals based onsaid complex weight coefficients to maximize the power of the totalcombined OFDM signals transmitted through said plurality of transmissionchannels.
 2. A communication device for transmitting OFDM signals in awireless communication system, the communication device comprising:antenna means having a plurality of antenna elements for transmittingsaid OFDM signals to a receiver device through a plurality oftransmission channels in said wireless communication system wherein eachof said OFDM signals is composed of a plurality of subcarrier signals;and means for generating weight coefficients which are applied to eachof said plurality of subcarrier signals and for controlling amplitudeand/or phase of each of said plurality of subcarrier signals as afunction of said weight coefficients, wherein said weight coefficientsare chosen to maximize the power of the total combined OFDM signalstransmitted through said plurality of transmission channels.
 3. Acommunication device for transmitting OFDM signals and for receivingtransmitted OFDM signals in a wireless communication system, thecommunication device comprising: antenna means having a plurality ofantenna elements for transmitting said OFDM signals to be sent through aplurality of transmission channels in said wireless communication systemwherein each of said OFDM signals is transmitted by using a plurality ofsubcarriers; and means for adjusting amplitudes and/or phases of each ofsaid plurality of subcarriers of the OFDM signals to maximize the powerof the total combined OFDM signals transmitted through said plurality oftransmission channels; wherein said power of the total combined OFDMsignals is expressed by elements of weight vectors and a Hermitianmatrix having elements of channel vectors, said weight vectors beingchosen in accordance with an Eigenvector of maximum Eigenvalue of saidHermitian matrix.
 4. A communication device for transmitting OFDMsignals and for receiving transmitted OFDM signals in a wirelesscommunication system, the communication device comprising: receivingmeans, having a plurality of antenna elements, for receiving a pluralityof OFDM signals transmitted by a number of transmission antennas via aplurality of transmission channels which correspond to a plurality ofsubcarriers in said wireless communication system; and means formaximizing the power of said plurality of received OFDM signals, whichare each weighted by respective weight vectors; wherein said power ofthe total combined OFDM signals is expressed by elements of weightvectors and a Hermitian matrix having elements of channel vectors, saidweight vectors being chosen in accordance with an Eigenvector of maximumEigenvalue of said Hermitian matrix.
 5. A communication device fortransmitting OFDM signals in a wireless communication system, thecommunication device comprising: antenna means having a plurality ofantenna elements for transmitting said OFDM signals propagated through aplurality of transmission channels in said wireless communicationsystem; means for obtaining channel information concerning phase shiftproperties of OFDM subcarrier signals transmitted through said pluralityof transmission channels; and means for controlling a phase of each ofsaid OFDM subcarrier signals in accordance with said channel informationto maximize the energy of the total combined OFDM subcarrier signalstransmitted through said plurality of transmission channels.